The tryptic peptide maps and amino acid compositions of tryptic peptides of lactate dehydrogenase (LDH) isozymes from mouse muscle, mouse heart, mouse testis, rat testis, human heart, beef heart, rabbit muscle and horse muscle, as well as alpha-glycerol phosphate dehydrogenase isozymes from Drosophila larva and adult, have been determined. The subunit A (muscle) and subunit B (heart) of mammalian LDH isozymes appear to be more closely related to each other than to subunit C (testis). The alpha-GDPH isozymes from Drosophila adult and larva appear to be coded by a single structural gene and different electrophoretic mobilities may be due to the post-translational modification. The structural characterization of the electrophoretic variants, F and UF isozymes of alpha-GDPH from Drosophila adults indicates the neutral amino acid substitutions as well as charge changes.